The present invention generally relates to a double monochromator and, in particular, relates to one having a grating pre-monochromator having an unsymmetrically ruled concave rotatable diffraction grating.
Conventional single monochromator assemblies generally include a light source unit comprising a light source and a concave mirror, via which the light source is imaged upon the entrance slit of the monochromator proper. In such a single monochromator, light having different wavelengths, a continuum for example, impinges upon the entrance slit. This, however, is not the only light which passes through the apparatus to emerge through the exit slit, as there is also a certain proportion of stray light which reaches the exit slit. The stray light can be due to scattering or the like and reaches the exit slit along other paths such that, in addition to the spectral band desired, a proportion of other wavelengths not desired also appears at the exit slit. This proportion may indeed by small. However, due to disadvantageous sensitivity characteristics of many photoelectric detectors, generally having a steeply dropping slope at the edge of the scanned wavelength range, the ratio of the stray light, or noise, signal to the useful signal may be intolerable.
The use of a pre-monochromator in place of a light source in a double monochromator is generally known from DE-AS 27 30 613. This pre-monochromator comprises a fixed concave mirror and rotatable flat diffraction grating. The light beam emitted by the light source is directed by the concave mirror onto the diffraction grating. A monochromatic light beam, the wavelength of which depends on the position of the rotatable diffraction grating, is thus directed upon the entrance slit of the main monochromator. The two monochromators are synchronously tuned via any conventional wavelength drive. Thus, only light of a desired wavelength range reaches the path of rays of the main monochromator. Thereby, the proportion of stray light reaching the exit slit is considerably reduced.
The pre-monochromator of DE-AS 27 30 613 is of particularly simple design. The concave mirror is disposed in the path of rays in front of the diffraction grating such that it directs a convergent light beam onto the diffraction grating. The pre-monochromator has a substantially larger spectral bandwidth than the main monochromator. The wavelength drive includes a setting drive causing rotation of the diffraction grating of the pre-monochromator which rotation is linear with wavelength. Due to this simple design, such a pre-monochromator in many cases may be provided instead of a lamp unit of a single monochromator without substantial variations of the basic design of the instrument. In many designs, however, this is not possible.
Monochromators having concave gratings are known from U.S. Pat. Nos. 3,909,134 and 3,930,728. In such monochromators, the grating is rotated about its nodal point. The monochromator includes fixed entrance and exit slits having particular positions with respect to the grating. The imaging of the entrance slit via the grating upon the exit slit is affected only slightly by the rotation of the grating.
In U.S. Pat. No. 3,909,134 the grating rulings are formed by the lines of intersection of the spherical calotte forming the concave grating with a plurality of quadrics of revolution, a focal point of each of these quadrics of rotation coinciding with the center of curvature of the spherical calotte. Therefore, the grating is symmetrical with respect to the distribution of the grating rulings. In U.S. Pat. No. 3,930,728, the grating rulings of a holographically manufactured grating are defined by interference lines produced on the spherical calotte forming the grating by two coherent light sources unsymmetrically disposed with respect to the grating in particular manner. This condition leads to an unsymmetrical distribution of the grating lines.